There are conventionally known systems for providing online games to user terminals such as personal computers, smartphones, or game-dedicated terminals connected to the system via a communication line such as the Internet (see, e.g., Japanese Patent Application Publication No. 2013-066524). For example, in a system for providing services generally called cloud gaming, a server performs arithmetic processing and image processing in accordance with progression of the game based on input information sent from a user terminal, and sends the result of the processing to the user terminal in the form of a video. For another example, in a system for providing services generally called multi-player online games, a server communicates with a game-dedicated terminal or a user terminal having installed thereon dedicated software by using a dedicated protocol, thereby providing games that can be simultaneously played by many users. Meanwhile, as smartphones are more commonly used, these online games are more often enjoyed with user terminals connected via networks under variable communication conditions such as mobile networks and having various and changing throughputs.